Titan's seasons Scientists have the first direct evidence of changing seasons on Saturn's largest moon, Titan.

The findings, published in the journal Nature, are shedding light on the structure of Titan's atmosphere, and could have implications for landing future probes on the moon's surface.

"It's the first time seasonal change has been seen on Titan," says planetary scientist Dr Elliot Sefton-Nash of University of California, Los Angles, who was among the international team of researchers.

Apart from Earth, the only other bodies in the solar system to have an atmosphere and a surface are Mars, Venus and Titan.

Titan's surface is shrouded by a toxic orange haze, formed from a combination of trace gases such as benzene and hydrogen cyanide, generated by photochemical reactions in the moon's upper atmosphere.

As sunlight heats the pole that is closest to the Sun, the haze circulates towards the other pole via Titan's middle atmosphere, and causes a concentration of trace gases at that pole, says Sefton-Nash.

"We can use those as tracers to trace the circulation of the atmosphere," says Sefton-Nash.

Titan's northern spring equinox - when both north and south hemispheres receive equal amounts of sunlight - was calculated to have last occurred in August 2009. One year on Titan lasts nearly 30 Earth years, so each season is seven years long.

This most recent reversal of seasons on Titan has been predicted by numerical simulations for 20 years, but only now have measurements made by NASA's Cassini spacecraft enabled it to be directly observed.

Infrared measurements

Using four years' of data from Cassini's Composite InfraRed Spectrometer, Sefton-Nash and colleagues have analysed the concentration of trace gases at Titan's south pole since the northern spring equinox.

"We see the concentration of these trace gases increasing at the south pole where all the air should be converging, so that backs up our story [that the seasons have reversed]," he says.

The researchers have also detected a "hot spot" above the south pole indicating that sinking air is being compressed and heated - even though winter temperatures on the ground would have been way below freezing.

"This was the opposite to what was seen before the spring equinox," says Sefton-Nash. "This is another indicator that the air is gathering at the south pole."

The findings are the first observation to confirm it is now indeed winter in Titan's southern hemisphere.

Scientists predict similar measurements to be made at the north pole after the southern spring equinox, but they will have to wait another 15 years to confirm this.

Higher circulation

The main atmospheric circulation on Titan was previously thought to reach 500 kilometres above the surface, but the latest evidence suggests it goes even higher, says Sefton-Nash.

He says the increase in trace gas concentration at the south pole was surprisingly large and suggests the gas could only have arrived there by sinking down from 600 kilometeres or more.

Sefton-Nash says the findings may have implications for future probes to Titan's surface.

"We know that there is probably a dynamic atmosphere up to 600 kilometres, so if we wanted to send a lander we have a better idea of what wind resistance and chemistry to expect when the spacecraft descends through the upper atmosphere," he says.

"Based on this research we might also expect to find exotic chemical compounds condensing, or even raining, on the surface at the winter pole, where trace gases are downwelling. Any future landers will no doubt want to analyse these."

Understanding Titan's atmosphere could also help in the study of atmospheric processes and climate on Earth and on extrasolar planets, says Sefton-Nash.

"Titan's atmosphere is similar to ours in a way, because it's made mostly of nitrogen. But the main difference is that Titan is so much colder because it gets very little sunlight, so the chemistry and dynamics are very different to Earth's," he says.

"Titan shows us how the atmospheres of very cold planets that are far away from their stars might behave. There may be many extrasolar planets like it."